Pressure sensor

ABSTRACT

Micromechanically producible capacitively operating pressure sensor, in which there is a diaphragm, formed by a diaphragm layer (5), on a silicon substrate (1) over a hollow (4) in an auxiliary layer (3), and in which there is, on the side of the diaphragm averted from the hollow (4) and at a distance therefrom, an electrode formed by an electrode layer (8) with recesses (9) therein as a counter-electrode to the electrically conductive diaphragm layer (5). Given a rise in the external pressure, it is possible by applying a voltage between the diaphragm layer (5) and the electrode layer (8) to prevent the diaphragm from deflecting in the direction of the substrate, and to determine the magnitude of the pressure from the voltage required therefor.

BACKGROUND OF THE INVENTION

The present invention relates to a micromechanically produciblecapacitive pressure sensor.

A closed chamber at a reference pressure is sealed by an elasticdiaphragm which is exposed to an external pressure. The electricallyconductive diaphragm forms a plate capacitor with the opposite side ofthis chamber. The diaphragm is deformed because of the pressuredifference between the external pressure and the internal referencepressure. The capacitance of this capacitor changes owing to the changein the distance between the diaphragm and the rear side of the chamber,which acts as a counter-electrode. The external pressure can bedetermined from this change in capacitance.

U.S. Pat. No. 5,095,401 by Paul M. Zavracky et al., describes pressuresensors on an SOI substrate, in which a hollow is produced by removing aregion, made from silicon oxide and produced by means of LOCOS, throughsmall openings in a layer applied thereupon. These openings aresubsequently sealed by oxidizing the adjoining silicon or by applying afurther layer made from silicon nitride, polysilicon or the like. Thispatent specification also describes the use of the method of productionspecified there for producing pressure sensors which measure in acapacitive fashion. A constituent of the diaphragm of this pressuresensor is a crystallized silicon layer which is used outside thepressure sensor to integrate electronic components, and is used in theregion of the diaphragm to form piezoelectric resistors.

SUMMARY OF THE INVENTION

It is the object of the present invention to specify a micromechanicallyproducible capacitive pressure sensor in which the problem of anonlinear dependence of the capacitance on the acting pressure iseliminated and to specify an associated production method which can becarried out in a BiCMOS process.

In general terms, the present invention is a pressure sensor as asemiconductor component having a hollow which is bounded on one side bya diaphragm formed essentially by an electrically conductive diaphragmlayer. On the side of the hollow averted from the diaphragm there is aregion which is doped in an electrically conductive fashion. There is anelectrically conductive electrode layer on the side, averted from thishollow, of this diaphragm at a distance from this diaphragm, in whichthis electrode layer is pierced by holes. These openings are of such anature, and this diaphragm can e deformed such that in the event thatwithin a prescribed interval there is a change in a pressure prevailingin a medium on the side, averted from this hollow, of this electrodelayer, it is possible to measure that change in a voltage, applied viaelectrical terminals to this diaphragm layer and this electrode layer orthis doped region, which is required to counteract a deformation of thisdiaphragm in an electrostatic fashion. There are contacts for theseelectrical terminals of this diaphragm layer, of this electrode layerand of the doped region.

Advantageous developments of the present invention are as follows.

There are recesses in the diaphragm layer in the region of thisdiaphragm which in each case pierce both surfaces of this diaphragmlayer. Material of a sealing layer applied to the diaphragm layer isintroduced into these recesses. This sealing layer does not cover theside of the diaphragm averted from the hollow.

The diaphragm layer is polysilicon.

The diaphragm layer is metal.

The electrode layer is formed by one or more metal layers.

The present invention is also a method for producing a pressure sensorthat has the following steps:

a) a layer provided for producing the hollow is produced above a dopedregion and the diaphragm layer is applied thereto;

b) a mask is used to produce in this diaphragm layer, in the region ofthe diaphragm to e produced, recesses of a size dimensioned for thefollowing steps c) and d);

c) a hollow is etched out in the layer provided therefor by using theserecesses;

d) a sealing layer is applied to the diaphragm layer such that therecesses are sealed without the hollow being filled up;

e) the electrode layer is applied and provided with the openings;

f) using these openings, material present between the diaphragm layerand the electrode layer is etched away in the region of the diaphragm tobe produced, and the means required to apply a voltage to the diaphragmlayer and the electrode layer are produced.

In a development of the present invention, a diaphragm layer is producedfrom polysilicon in step a), and dopant is implanted in a following stepin order to render the diaphragm layer electrically conductive.

In a further development of the present invention, a spacer layer isapplied between steps d) and e), and this spacer layer is etched away instep f) in the region of the diaphragm to be produced.

The pressure sensor according to the invention achieves the object setby using electrostatic compensation to prevent movement of the diaphragmin the event of a change in the acting external pressure. The pressureis determined from the magnitude of a voltage required for thiselectrostatic compensation. Because the diaphragm is not deflected,there is no nonlinearity. Consequently, there is also no need tostructure the diaphragm in a special way or to limit the maximumpossible deflection of the diaphragm. The electronic evaluation of themeasuring signals is likewise simplified. During production, the designaccording to the invention, which employs surface micromechanics, makesuse only of process steps which are a standard feature of a BiCMOSprocess. Additionally required process steps in which masks are used canbe performed in such a way that they are as far as possible compatiblewith the remainder of the process. It is therefore possible to realizesensor components and circuit components simultaneously in a simple wayon one chip.

The mode of operation of the pressure sensor is based on electrostaticforce compensation (Force Balancing Sensor, FBS). In this case, it isnot the deflection of the diaphragm which serves as a measure of thepressure, but the force which is required to keep the diaphragm in therest position, and which is determined indirectly from the voltage whichthis force causes in an electrostatic fashion. For the pressure sensorto be able to be used at a higher pressure than the reference pressure,it is necessary that an electrostatic compensation force can act on thediaphragm in a direction opposite to the acting external pressure. Forthis purpose, the diaphragm is designed to be electrically conductive,and there is a pierced counter-electrode on the outside of the pressuresensor above the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

The single FIGURE depicts the pressure sensor of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A diaphragm layer 5, a sealing layer 7, an electrode layer 8 and furtherlayers 10, 11 are represented in the FIGURE on a substrate 1 having aconductive region 2, which is constructed on one surface and is providedas a bottom counter-electrode, and having an auxiliary layer 3, whichcan be, for example, an upper component layer of the substrate 1 whichis oxidized in parts by means of LOCOS. Located in the auxiliary layer 3is a hollow 4 which is evacuated or is filled with air or another gas orgas mixture which is at the reference pressure. Recesses 6 which arefilled with material of the sealing layer 7 removed in the region of thediaphragm 12 are located in the diaphragm layer 5. An interspace whichpermits the diaphragm to be deflected in both directions perpendicularto the diaphragm plane is located between the diaphragm 12 and theelectrode layer 8. Recesses 9 which are not sealed are located in theelectrode layer 8 in order that an external pressure can act on thediaphragm 12 formed by the component of the diaphragm layer 5 locatedabove the hollow 4. The further layers 10, 11 present above theelectrode layer 8 are removed in the region of the diaphragm 12.

The recesses 9 in the electrode layer 8 each pierce both boundarysurfaces of the electrode layer 8 and are dimensioned and arranged suchthat, in the event of a change in the external pressure, due to flowthrough these recesses 9, this change in pressure is communicated evento the diaphragm 12 in a time which is sufficiently short for theintended use of the pressure sensor. The doped region 2 can be omittedin the case of special embodiments provided for measuring an increasedpressure. The electrical terminals at the conductive layers 5, 8 arelocated to the side and are not illustrated in the FIGURE. Contacts onthe conductive region 2, the diaphragm layer 5 and the electrode layer 8are applied in the manner known per se.

A substrate 1 made from silicon is used, for example, in producing thispressure sensor. The conductive region 2 can be formed on a top side bydiffusing in dopant. The auxiliary layer 3 is preferably produced bylocal oxidation of the silicon by means of LOCOS. However, instead ofthis it is possible to apply a separate layer as auxiliary layer 3. Forexample, polysilicon which is applied together with the gate electrodesfor simultaneously produced MOSFETs and is subsequently doped in anelectrically conductive fashion can be used for the diaphragm layer 5.Instead of this, a metal layer or sequence of layers of various metalscan be used for the diaphragm layer 5, such as are applied, for example,as metallic coating for the wiring of electronic circuits produced onthe same chip.

The recesses 6 in the diaphragm layer 5 are produced by means of aperforated mask, and the hollow 4 is produced through these recesses 6by etching out the auxiliary layer 3 or removing the oxide previouslyproduced by means of LOCOS. Thereafter, the sealing layer 7 is applied,and this can be, for example, a planarizing layer made from dielectric.The recesses 6 can be dimensioned such that they are sealed with thematerial of the sealing layer 7 without the hollow 4 being filled up inthe process. The electrode layer 8 is then applied over the entiresurface of the sealing layer 7. The electrode layer 8 can expediently beformed, for example, by one or more metal layers of the levels ofmetallization provided for the electrical wiring.

The recesses 9 are produced in the electrode layer 8 by means of afurther mask, and the material of the sealing layer 7 is removed fromthe surface of the diaphragm through these recesses 9 in the region ofthe diaphragm 12 to be produced. In this process, the time of theetching process is limited such that the recesses 6 in the diaphragmlayer 5 remain sealed by remaining material of the sealing layer 7. Thefurther layers 10, 11 are, for example, passivation layers or insulationlayers which are applied in the course of the overall process. Thesefurther layers 10, 11 are removed from the top side of the electrodelayer 8 in the region of the diaphragm 12, the recesses 9 being openedin the electrode layer 8. If the sealing layer 7 cannot be applied at athickness required for the capacitance which is adequate for theenvisaged measuring range, a further layer can be provided as spacerlayer between the sealing layer 7 and the electrode layer 8.

In the case of this pressure sensor, the measurement result does notcontain a nonlinearity in the capacitance as a function of the diaphragmdeflection when a voltage for producing an electrostatic restoring forceis applied to the counter-electrode formed by the electrode layer 8 or,if appropriate, to the doped region 2 formed in the substrate 1. Thesquare of this applied voltage is proportional to the acting pressure(more accurately: pressure difference between the pressure in the hollow4 and the external pressure). This produces an exactly defineddependence of the measuring signal on the pressure difference to bemeasured. The nonlinearities occurring in the case of conventionalcapacitive pressure sensors therefore do not occur and do not have to beeliminated in an expensive way, for example by subsequent evaluation inthe electronic circuit.

The invention is not limited to the particular details of the method andapparatus depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedapparatus without departing from the true spirit and scope of theinvention herein involved. It is intended, therefore, that the subjectmatter in the above depiction shall be inter-pretend as illustrative andnot in a limiting sense.

What is claimed is:
 1. A pressure sensor comprising:a semiconductorhaving a doped region, a hollow area in a further region over the dopedregion, a diaphragm layer having first and second sides with the firstside adjacent the hollow area, and an electrically conductive electrodelayer over the second side of the diaphragm; the electrode layer beingpierced by holes; the diaphragm being deformable such that when there isa change in a pressure prevailing in a medium on the second side of thediaphragm, it is possible to measure a change in a voltage, applied viaelectrical terminals to the diaphragm layer and one of the electrodelayer or the doped region, which is required to counteract a deformationof the diaphragm in an electrostatic manner; and contacts connected tothe electrical terminals.
 2. The pressure sensor as claimed in claim 1,wherein the diaphragm layer has recesses, each of the recesses piercingboth first and second sides of the diaphragm layer, wherein a sealinglayer is located between the diaphragm layer and the electrode layer,wherein material of the sealing layer applied to the diaphragm layer isintroduced into the recesses, and wherein the sealing layer does notcover the second side of the diaphragm layer in an area for therecesses.
 3. The pressure sensor as claimed in claim 1, wherein thediaphragm layer is a polysilicon layer.
 4. The pressure sensor asclaimed in claim 1, wherein the diaphragm layer is a metal layer.
 5. Thepressure sensor as claimed in claim 1, wherein the electrode layer hasat least one metal layer.